1. Field of the Invention
The present invention generally relates to apparatuses and methods for processing a moving image, and more particularly, to an apparatus and method for processing a series of interlaced image.
2. Description of the Related Art
Recently, JPEG2000 has been known as a compression and coding method suitable for processing high-definition images. Further, there is also a standard known as Motion JPEG2000 that displays a moving image by successively reproducing still encoded according to JPEG2000. 
The inter-field movement (change) of an object, which is a parameter that does not pertain to a still image, exists in a series of interlaced images captured by a video camera, comparing the interlaced image of a field with that of the previous field. There have already been proposed Motion JPEG2000-compliant moving image processing apparatuses that detect the movement speed of an object within a frame using the above-described parameter of movement, and adaptively perform a compression and coding process. Japanese Published Examined Patent Application No. 4-77517 (corresponding to Japanese Laid-Open Patent Application No. 63-148790) discloses one of such moving image processing apparatuses.
The Motion JPEG2000-compliant moving image processing apparatuses perform the operation of obtaining an inter-field difference in the image data of an object and calculating the movement speed of the object based on the obtained difference data. Accordingly, a large amount of data should be processed so that a large amount of time and a large amount of memory capacity are required for the operation.
There are a variety of well-known conventional image processing apparatuses that convert image data into frequency-region coefficients by discrete cosine transform (DCT) or two-dimensional wavelet transform (DWT), quantize the coefficients frequency by frequency, and perform entropy coding on the quantized coefficients. For instance, Japanese Laid-Open Patent Application No. 8-186816 discloses an image processing apparatus that converts a quantization step size employed in the above-described quantization into a unit of image quality control (for instance, the unit of a sub-band in the case of using wavelet transform) in order to increase the amount of data compression while preventing the degradation of image quality.
This image processing apparatus, which uses two-dimensional DWT, sets the quantization step size for quantizing the wavelet coefficients obtained by the DWT so that the  quantization step size is the largest for the HH sub-band, the second largest for the HL sub-band, and the smallest for the LH sub-band with respect to a luminance signal, and is the largest for the HH sub-band, the second largest for the LH sub-band, and the smallest for the HL sub-band with respect to a color difference signal.
The contents of the adjustment of the quantization step size are obtained based on experimental data obtained from experiments using “Mobile & Calendar,” which is a standard MPEG image and are not specified based on the characteristics of a halftone image.
Further, a non-interlaced image having the interlaced images of two fields successively captured by a video camera at every interval of 1/60 second includes a comb-shaped pixel offset corresponding to the horizontal movement speed of an object. The above-described image processing apparatus does not take into consideration the fact that the comb-shaped pixel offset changes with the movement speed of the object. Accordingly, depending on the movement speed of the object, an originally single vertical line may become two separate lines or the outline of a reproduced image may include horizontal blurring, thus causing a great degradation of image quality.
Recently, improvements in image input and output technologies have greatly increased demand for high-definition images. In the case of digital cameras as image input apparatuses, for instance, high-performance charge coupled devices (CCDS) having 3,000,000 pixels or more have been reduced in price to be widely used in digital cameras in a popular price range. It is expected that products employing CCDs having 5,000,000 pixels or more will be commercially available in the near future. It is expected that this trend toward an increasing number of pixels will continue for a while.
On the other hand, there have also been remarkable developments in the high-definition  property and significant progress in the price reduction of image output apparatuses and image display apparatuses such as hard-copy apparatuses including laser printers, inkjet printers, and sublimation-type printers, and soft-copy apparatuses including flat panel displays made of CRTs, liquid crystal displays (LCDs), and plasma display panels (PDPs).
Due to the introduction of these high-performance, inexpensive image input and output apparatuses to the market, high-definition images have become popular. As a result, it is expected that there will be an increasing demand for high-definition images in various fields in the future. Actually, the developments in technologies related to PCs and networks including the Internet have accelerated such trends at an increasing rate. Particularly in recent years, mobile equipment such as mobile phones and notebook personal computers has become so popular that opportunities to transmit or receive high-definition images anywhere through communication means have increased rapidly.
It seems inevitable that, with these background trends, demand for improvement in the performance and multi-functioning of image compression and/or decompression technologies will become stronger in the future so that processing of high-definition images can be facilitated.
Therefore, in recent years, a new image compression method called JPEG2000, which can restore with high quality an image compressed at a high compression rate, has been standardized as one of image compression techniques satisfying such demand. According to JPEG2000, by dividing an image into rectangular regions called tiles, compression and decompression can be performed on the image with a small memory capacity. That is, each individual tile serves as a basic unit in performing compression and decompression processes, So that the tiles can be subjected to the compression and decompression processes independent of  one another.
Further, such single-frame JPEG2000 images may be successively displayed at a predetermined frame rate (representing the number of frames reproduced per unit of time) as a moving image. There is an international standard called Motion JPEG2000 for successively displaying single-frame JPEG2000 images as a moving image.
Japanese Laid-Open Patent Application No. 2001-309381 discloses the technique of compressing and encoding image data using DWT as Motion JPEG2000. According to this technique, not only pixel values are compressed and encoded by DWT, but also the images of different frames are correlated in order to eliminate the redundancy of moving image data in the case where no image movement between the frames occurs. As a result, the rate of data compression can be further improved. However, in order to obtain the correlation between the frames, this technique requires the complicated processing of decoding and inversely quantizing the encoded orthogonal transform coefficients, thus requiring more processing time. Further, this technique requires a memory capacity for storing a preceding one of the frames used for obtaining the inter-frame correlation. 